Old 24-07-2021, 10:39 AM
stephen2615 (Stephen)
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Location: Canberra, Australia
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CST back focus question


I have used a refractor for some time and just bought a Celestron EdgeHD 9.25. Back focus on the refractor was absolutely critical. I expect to have a problem getting exactly the right back focus with the CST and mentioned that to a vendor. He said that back focus with something like the EdgeHD series is not as critical as with a refractor. I might be out by two mm with my current equipment and he said, it would not really impact the edges compared to the refractor.

Is that true? If so, can someone explain in layman's terms why it would be so.


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Old 24-07-2021, 12:04 PM
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Camelopardalis (Dunk)
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Location: Brisbane
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IIRC the spherical aberration with the EdgeHD series is minimised at 146mm on the 9.25 and upwards. If you stray inward or outward from that, it increases at the edge of the field, but not very rapidly.

I remember seeing plots somewhere, a few years ago, and thought that it was a lot less dramatic than expected, even over deviations of 5-10mm from stated and it's mostly noticeable off-axis and exacerbated with a full-frame camera sensor.

In practice, you might see stars looking more fuzzy than sharp, as you stray away from the recommended back focus, but my take on that is that given the focal lengths of the 9.25 and above, you're prolly going to get some ambiguity in the stars because of the pesky atmosphere

With field flatteners used in refractors, incorrect back focus is very noticeable except with the smallest of sensors, and coma correctors used in newtonians tend to be pretty fussy too.
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